<!DOCTYPE html>
<script src="../../resources/run-after-layout-and-paint.js"></script>
<script>
if (window.testRunner)
  testRunner.dumpAsText();
onload = function() {
  // Expose left part of the inner horizontal scrollbar first.
  document.getElementById('outer').scrollTop = 250;
  runAfterLayoutAndPaint(function() {
    // Expose the forward track and forward button of the inner horizontal scrollbar.
    document.getElementById('outer').scrollLeft = 250;
  }, true);
};
</script>
Tests invalidation and painting of scrollbar parts of a scroll area contained by another scroll area, when scrolling of the outer scroll area exposes a new port of the inner scrollbar. Passes if no assertion failure.
<div id="outer" style="position: absolute; top: 100px; left: 50px; width: 200px; height: 200px; overflow: scroll">
  <div id="inner" style="width: 400px; height: 400px; overflow: scroll">
    <div style="height: 500px; width: 100%; border: 1px solid black">PASS</div>
  </div>
</div>
